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Electrolytic capacitor 400 V or 450 V in inverter ?

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anotherbrick

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hello dear forum,

I am designing an 3 KW half bridge inverter for 28 KHz output

the inverter will run on the single phase 220 VAC mains

I will use 1200 uF electrolytic bypass capacitor for the DC-link

my question ;

shall I use 400 V or 450 V for capacitor nominal voltage ?

thank you
 

Hi,

220V AC gives +/-311Vp

Now the question is: how reliable is the 220V. In case it is +20% then the peak voltage is 373V.

Overtones may increase peak voltage additionally, but it is not very likely.

But what about short erroneous overvoltage? Is there an overvoltage protection circuit?

****
I´m designing reliable industrial electronic equipment, I´d go for 450V ones.

Klaus
 
Depends what is putting charge in to the dc link cap and how variable that is.
As you know, your inverter is putting power into the mains.
Something else is putting charge into the DC link cap
What is this?
 
KlausST said:
Hi,

But what about short erroneous overvoltage? Is there an overvoltage protection circuit?

****
I´m designing reliable industrial electronic equipment, I´d go for 450V ones.

Klaus
Click to expand...

I put a varistor in parallel with the caps - is it ok ?

- - - Updated - - -

treez said:
Depends what is putting charge in to the dc link cap and how variable that is.
As you know, your inverter is putting power into the mains.
Something else is putting charge into the DC link cap
What is this?
Click to expand...

I am not putting power into the mains - I get power from the mains and invert it to a frequency of 28 Khz to drive ultrasonic transducers
 

Hi,

Varistor ... combined with a fuse in series ... maybe.

Mind that a varistor isn´t that precise. Look into the datasheet.


Klaus
 
Well in your case then you shoudl be ok with 400v cap.
Not varistor. They only for very short microsecond spikes really...they wear out and short eventually...theycan only quench so many times.

- - - Updated - - -

but check the for any overvoltage ringing on the cap at switch on, when the ringing with the lc imput filter can be severe with the inrushing current.
switch it on at the mains peak and see how high it rings, if it rings above 400v, then you need cap >400v
 
You might want to measure the RMS ripple current into that 1,200uF capacitor, both at 120 Hz and 28Khz.
3KW is getting fairly high. Temperature rise of the capacitor (and life) may be a concern.

If you need a higher ripple current rated capacitor, a higher voltage rating might be one possible solution to that.
 
Does it make that much of a difference in terms of cost or size?

400 and 450 are both very common in exactly this application meaning parts are well optimized for both cost and size at these voltages and there are plenty of options available.

Normally I would think 450V is already cutting it close to 373 peak, but it's pretty common to cut it close in this application from what I've seen.
 
Are you able to simulate this in order to gain an understanding of the Harmonic current in the Capacitor? That is normally of great concern in AC motor drives as that directly affects the lifetime of the product. Capacitor voltage rating on a 220V single phase input drive is almost always 400V as that is generally covered by the mains tolerances. Generally there additional low ESR Caps in parallel in order to soak up transients.
 
You must factor in the margins: 220V +/- 10%??

Your design terms: input effective volts:

Your capacitor ratings margins (on voltage): 450 +/- 10%??

If you are planning for a reliable design, plan for the worst case.
 
3 Kw output.......... you are going to employ power factor correction?
I would strongly suggest that you consider doing so.

Since PFC circuits employ a boost topology, your voltage will become larger. That is the reason you see 450 volt capacitors.
 

Agree with schmitt trigger PFC at that power level off single phase would be a good idea. This does also lead to higher DC voltage but you can set that in the control loop of the PFC controller. DC bus has to be higher than the peak of the mains for it to work.
 

I am not sure about the tolerances for the voltage ratings for electrolytic capacitors.
 

I am not sure about the tolerances for the voltage ratings for electrolytic capacitors.
Click to expand...
????

Maximum ratings have no tolerance by nature. The capacitor might have an additional surge voltage specification, e.g. +10 %. Read the datasheet. But that's not a tolerance.

Most 230V operated instruments with direct rectifier use 350V capacitors, 400 V would give some margin for unusually hight overvoltage. 450V is typically used for boost PFC input stages.

Surge voltage VS
The surge voltage is the maximum voltage which may be applied to the capacitor for short periods of time, i.e. up to 5 times for 1 minute per hour. IEC 60384-4 specifies the surge voltage as follows:
for VR > 315 V: VS = 1.15 · VR
for VR > 315 V: VS = 1.10 · VR
Click to expand...
 
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